1995 Oldsmobile 98 seem to miss at a steady speed

Have replaced wires and plugs at 118,000 miles, car is maintained well, has been running fine. Car idles good, good acceleration but it seems to have a slight miss in it at a steady speed, more so from 40 mph on up, gas mileage doesn't seem to be effected, if so, not very much. Have been getting around 18-22 City driving and 24 or so on the Hway. It was running rough at times before the car would get warmed up but a mechanic had 2 sensors he had off of another motor that he had used. So, I believe he had put a throttle positoner sensor and a MAF Sensor in. The car idles fine now, just have the trouble with that small miss. Appreciate any help. Would U please advise me the where abouts of the sensors. The car has a 3.8 motor. Nice car. Plan on keeping it. Thanks again for any info.